1. Field of the Invention
The present invention relates to the structure of a supporting portion at which a fork stem is supported on a head pipe of a bicycle body and a process of manufacturing such a supporting portion.
2. Disclosure of the Prior Art
FIG. 5 is a cross sectional view showing a conventional structure of a supporting portion for supporting a fork stem on a head pipe of a bicycle body.
The upper and lower ends of a head pipe (15) fitted on a fork stem (13) of a front fork (12) are both enlarged in a flared fashion to respectively form a ball bearing accommodating portion (17). Each ball bearing accommodating portion (17) accommodates a ball bearing (2) of a cartridge type.
Overlaid on the ball bearing (2) mounted on the upper end of the head pipe (15) is a ring-like cover (23). A plug-in cylinder (24) positioned under the cover (23) is fitted in an inner ring (22) that constitutes the ball bearing (2), with the fork stem (13) of the front fork (12) passing through the inner ring (22).
In this structure, the front fork (12) rotates relative to the head pipe (15), being supported by the upper and lower ball bearings (2), which permits handle operation.
The conventional supporting portion described above, however, has to prevent backlash in the ball bearings (2) by bringing the inner circumferential shape of each ball bearing accommodating portion (17) into precise registration with the peripheral shape of an outer ring (21) of each ball bearing (2). Hence, the above conventional supporting portion disadvantageously needs to be finished by high precision cutting of the inner circumferences of the enlarged portions after enlarging the openings at both ends of the head pipe (15), so that considerable labor is involved in the manufacture of the supporting portion for the fork stem.
An object of the invention is to enable easy manufacture of a supporting portion for a fork stem.
The above object can be accomplished by a first aspect of the invention which provides a supporting structure for a fork stem used in a bicycle, the supporting structure comprising:
a head pipe which is formed from a tubular pipe having a constant diameter and which has ball bearing accommodating portions each located at a position interior to an opening end of the head pipe relative to an axial direction and
ball bearings each of which has steel balls and first and second rings for holding the steel balls therebetween and supports the fork stem passing through the second ring,
wherein said ball bearing accommodating portions are each composed of (i) a support ring supported in a push-in inhibited fashion by an annular face that is formed on the inner circumferential surface of the head pipe so as to be perpendicular to an axial direction and (ii) one of the ball bearings with the first ring being supported in a push-in inhibited fashion by the support ring.
In the above supporting structure, the following procedure is taken for incorporating each ball bearing in the head pipe. The support ring is supported in a push-in inhibited fashion on the annular face formed at a position interior to an opening end of the head pipe relative to an axial direction. Then, the ball bearing is inserted into the head pipe from its opening end and the first ring of the ball bearing is supported in a push-in inhibited fashion by the support ring, whereby the ball bearing is incorporated in the fork stem. Thereafter, an operation for making the fork stem pass through the second and first rings of the ball bearing is carried out to assemble the fork stem into the head pipe so as to be freely rotatable.
The present invention has the following inherent effect. The head pipe can be produced by simply forming annular faces on the inner circumferential surface of the head pipe, the annular faces being used for assembling the support ring. With this arrangement, there is no need to form flared enlarged portions at both ends of the head pipe and to precisely cut each enlarged portion for registering its inner circumferential shape with the peripheral shape of each ball bearing. Accordingly, the supporting portion for the fork stem can be easily manufactured.
The above object can be also accomplished by a second aspect of the invention which provides a process of manufacturing a supporting portion for a fork stem used in a bicycle, the process comprising:
the first step of cutting, at preset intervals, a long tubular pipe having a constant diameter to form a workpiece;
the second step of cutting the inner circumference of the workpiece at its both ends to form annular recesses, thereby forming a head pipe;
the third step of respectively fitting the peripheral portions of support rings into the annular recesses thereby attaching the support rings to the head pipe;
the fourth step of respectively bringing, from either or both of the ends of the head pipe, ball bearings into engagement with respective portions of the associated support rings, the portions of which project from the annular recesses, thereby allowing the ball bearings to be accommodated within the head pipe; and
the fifth step of making the fork stem pass through the ball bearings so that the fork stem is freely rotatably supported on the head pipe by the ball bearings.
In the above method, a long tubular pipe is cut at regular intervals corresponding to the length of a head pipe to be produced, thereby forming a workpiece (the first step). The inner circumference of the workpiece at both ends is then cut to form annular recesses (the second step). Next, the support rings are inserted into the head pipe with their peripheral portions respectively fitted into the annular recesses formed on the inner circumference of the head pipe at the both ends, thereby respectively attaching the support rings to the annular recesses (the third step). Thereafter, ball bearings are inserted into the head pipe from both ends thereof to engage the periphery of each ball bearing with the inner circumference of each support ring, thereby allowing the ball bearings to be respectively supported on the support rings (the fourth step). Finally, the fork stem, which has been inserted into the head pipe from its end, is made to pass through the ball bearings (the fifth step). With this procedure, the fork stem can be freely rotatably attached to the head pipe through the ball bearings.